As Mo Farah crossed the line at the head of the field in the 10,000 metres,
the look on his face was one of total astonishment.

His eyes spreading wide, he glanced over his shoulder to confirm that he was indeed first, that the noise threatening to demolish half of east London really was in acknowledgment of what he had just achieved.

Slapping his head as if desperate to force the images into his mental cinema, he fell to the track, swooning in his moment.

You could appreciate his sense of shock. Britons don’t win Olympic gold in distance running. No-one from these islands has ever before picked up the winner’s medal at 10,000metres. And Farah is from these islands.

Sure, he may have been born in Mogadishu. But this is no Plastic Brit, seeking a passport of convenience. He had never run a competitive yard before he came here as an eight year old with his parents from war-stricken Somalia. It was in Britain that he found his vocation. It was on the streets of London that he was formed as a competitor.

“If I hadn’t come to England, I wouldn’t be a runner,” he once told me. “I wouldn’t be an athlete, I wouldn’t be in the Olympics. Nobody runs in Somalia. It’s not like Ethiopia or Kenya where everyone runs.

"I went back there for a visit in 2008, I went out for a run and people stopped me to ask me what I was doing. You’re running? Are you mad? All the little kids were laughing at me, like: this guy’s crazy.”

And it was to thank the country that gave him shelter and purpose that he set out to win this race on Saturday night. Fully 25 laps of the track would constitute the most gruelling thank-you note anyone could deliver.

25 laps moreover against 29 runners including contenders as accomplished as Kenenisa Bekele of Ethiopia plus Wilson Kiprop and Moses Masai of Kenya. No-one was going to gift him a victory.

But he arrived in the stadium enthused. He had watched Jessica Ennis win the heptathlon before him, he had heard the noise this crowd could generate, he had drawn energy from it.

As he walked to the start line for the most significant engagement of his life, he passed Ennis, beaming in victory, and the pair exchanged words. She told him to go out there and win it.

If he were carrying the hopes of his adopted country on his slim shoulders, however, Farah looked the most relaxed person in the stadium. As the runners stood on the start line, the Briton - resplendent in his fluorescent green shoes - cupped his ears to the ovation that greeted his name.

For all the hours of training, all the miles at altitude, all the weeks of mental and physical conditioning, nothing could have prepared him for the sound that greeted his appearance.

As if this were not enough, when the race began the long jump was reaching its finale. The throaty growl for Greg Rutherford’s gold rush swooshed across the track.

It became immediately clear what the pattern of this race was to be. Farah was on his own. It was like the Tour de France out there. The Briton was ganged up on by Kenyans, Ethiopians and Eritreans who, sharing the lead, working in teams, took it in turns to try to inflict pain on their rival, to draw his strength.

Farah had no posse of domestiques to help him out. His old training partner Chris Thompson was soon fulfilling the traditional role of the Briton in races like these, bravely plodding at the back, too far off the pace to assist Farah. But as the laps ticked by, Farah remained aloof from the scrappy kick and rush going on around him.

The lead was changing constantly. For one lap it would be a pair of Ethiopians up front, the next the Eritreans had elbowed forward. Farah, held back, looking as relaxed as he had at the starting gun, his leggy stride unperturbed and unflustered.

And at the bell he made his move. As he moved past the Bekele brothers and stretched his gazelle-like stride, the volume dial was switched up once more. The roar as he hit the front with 400metres to go was astonishing. Even those watching from the restaurant at the top of the Orbit sculpture were yelling.

“My legs were getting tired,” Farah explained of that last lap. “I needed to dig in and the crowd just gave me that boost.”

It was enough to propel him round the final bend in front. The field behind kicked, but Farah was not going to lose it now. He increased his stride, ate up the track, glided towards the line. Behind him his American training partner Galen Rupp surged past the rest into second place.

Suddenly African dominance - which had been demonstrated in an Ethiopian/Kenyan carve-up in the womens’ 10,000 metres the day before - was being shattered before our disbelieving eyes.

Farah did it 27min 30.42sec, nowhere near his best. But races at this distance are not just won on speed. They are won by the application of tactics, of nerve and determination. Farah had all that. And more. He had purpose.

As he celebrated with his daughter and his heavily pregnant wife down on the track you got a glimpse of what that purpose was. Britain had given him the chance to lead a normal, peaceful family life. And at last, in the most delirious of settings, he had found the means to deliver the most pertinent vote of thanks.